A wide variety of industrial instruments have been developed using flexible metal or non-metallic diaphragms for diverse purposes. A common such use is in the measurement of process fluid pressures, where the diaphragm is a seal that serves to isolate the process fluid from the instrument fluid and pressure instrument mechanism while simultaneously sensing and transmitting the fluid pressure to the sensing element of the pressure instrument.
In designing such pressure-responsive diaphragms, two generally incompatible characteristics are sought to be maximized--sensitivity and durability. Sensitivity--as measured by the force or pressure necessary to temporarily deform or deflect the diaphragm--directly affects the accuracy of the pressure measurement. Durability affects both accuracy and practicality; if the diaphragm lacks durability, it can become permanently deformed, causing inaccuracy or rupture, causing damage to the recording mechanism. The less durable a diaphragm is, the more often it must be replaced, causing costly and undesirable "down time".
Increased durability is achieved at the expense of sensitivity. To achieve accuracy coupled with durability, diaphragm designers have focused on structures whose effective surface area remains substantially constant over a moderately wide range of deflection and whose surfaces contain corrugations or convolutions to permit deflection of the diaphragm with minimal stress on the diaphragm fibers. The diaphragm material must, of course, be thin enough to flex easily, yet thick enough to resist corrosion and permanent deformation or rupture. When made of metal, a diaphragm for a pressure recording instrument is typically 0.004 to 0.005 inches thick and about 2 to 4 inches in diameter.
Whatever the exact thickness or composition of the diaphragm material, a flat diaphragm is subject to two distinct types of tensile stress as the diaphragm is deflected under pressure. One of these is radial stress; the other is circumferential stress (commonly called "hoop stress"). These stresses occur because, as pressure is applied to one side of a flexible diaphragm, the diaphragm will flex--or "dome out"--in the opposite direction, thereby assuming a convex or parabolic profile. The diaphragm material will be stretched both along its diameter (radial stress) and circumferentially. It is evident that the surface area of the diaphragm in its domed position is greater than the surface area of the diaphragm in its rest position. This enlargement of the diaphragm surface stretches the diaphragm fibers circumferentially, thereby creating "hoop stress".
The prior art has dealt with radial stress by shaping annular corrugations into the diaphragm structure, as shown in Mounteer, U.S. Pat. No. 3,079,953 and Zavoda, U.S. Pat. No. 3,645,139. These corrugations, or convolutions, are concentric with the circumference of the diaphragm and facilitate the radial displacement of the diaphragm material by providing additional material that is predisposed to bending in a radial direction.
As in Anderson, U.S. Pat. No. 128,452 and St. Clair et al., U.S. Pat. No. 3,340,734, the prior art has reduced "hoop stress" by shaping radially extending ribs or "spokes" into the diaphragm structure. These spokes facilitate the circumferential flexing of the diaphragm by providing additional material that is predisposed to bending in a circumferential direction (i.e., in a direction generally perpendicular to the radius).
Although a "herring bone" design was made known to the art by Gray, U.S. Pat. No. 2,874,569, prior designers have failed to reduce both radial stress and circumferential stress with one design integrating multiple annular corrugations and spokes. This is because the spokes, or radial convolutions, have been constructed as radial stiffening elements that will offset or neutralize the radial flexing effects of the annular corrugations. An example of this is Mounteer, U.S. Pat. No. 3,079,953. Moreover, prior structures have been limited to single, rather than multiple, annular corrugations when the stiffening ribs or spokes are imposed on the annular corrugation. Examples include Kelley, U.S. Pat. No. 1,793,621 and Bowditch, U.S. Pat. No. 3,187,641. Such a single corrugation, which is trough-shaped rather than wave-shaped, is obviously adaptable to ribbing, but only facilitates diaphragm movement over a narrower range and with less sensitivity of response than would be possible using wave-shaped (multiple) corrugations. In effect, the distortion of the preformed trough introduces a compressive stress as the diaphragm is already "domed out" in a predetermined direction, in contrast to the tensile stresses resulting from displacement of the discal or planar form of diaphragm.